One of the systems being considered after the third generation is an Orthogonal Frequency Division Multiplexing (OFDM) system that can diminish inter-symbol interference effect owing to low complexity. OFDM converts serial input data into N parallel data and transmits the parallel data on N orthogonal subcarriers. The subcarriers maintain orthogonality in frequency dimension. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access method that realizes multiple access by independently providing each user with some of available subcarriers in a system using OFDM as a modulation scheme.
One of major problems of the OFDM/OFDMA system is that PAPR (Peak-to-Average Power Ratio) can be very high. A problem of PAPR is that peak amplitude of a transmission signal is much higher than average amplitude, which is due to the fact that an OFDM symbol is overlap of N sinusoidal signals on different subcarriers. PAPR is particularly related to capacity of a battery and causes a problem in a user equipment that is sensitive to power consumption. In order to reduce power consumption, the PAPR needs to be lowered.
One of systems proposed to lower PAPR is Single Carrier-Frequency Division Multiple Access (SC-FDMA). SC-FDMA is a form that combines Single Carrier-Frequency Division Equalization (SC-FDE) scheme and Frequency Division Multiple Access (FDMA) scheme. SC-FDMA has a characteristic similar to that of OFDMA in that it modulates and demodulates data in time and frequency domains using Discrete Fourier Transform (DFT). However, since PAPR of a transmission signal is low, SC-FDMA is advantageous for saving transmission power. Particularly, in relation to use of battery, SC-FDMA is advantageous to uplink through a user equipment sensitive to transmission power communicates with the base station.
An important point when the user equipment transmits data to the base station is a wide coverage, in which bandwidth of data is not so wide, but power can be concentrated. An SC-FDMA system lowers variance of a signal and thus has a coverage further wider that those of other systems when using the same power amplifier.
In order to implement a variety of transmit or receive techniques for transmitting high-speed packet, transmission of control signals in time, space, and frequency domains is an indispensable element. A channel that transmits control signals is referred to as a control channel. There may be various kinds of uplink control signals, such as a scheduling request signal transmitted by a user equipment to the base station to allocate uplink radio resources, an Acknowledgement (ACK)/Negative-Acknowledgement (NAK) that is a response to uplink data transmission, Channel Quality Indicator (CQI) for indicating channel quality of downlink, Precoding Matrix Index (PMI), Rank Indicator (RI), and the like.
The scheduling request signal is carried by a presence or absence of transmission (or on/off keying) of a control channel, which is used for a user equipment to request a base station to perform scheduling for the user equipment. So when the user equipment needs to transmit uplink data to the base station, the scheduling request signal should be preliminarily transmitted. In most cases where the transmission interval of the scheduling request signal is not overlapped with those of an ACK/NACK signal and CQI, the scheduling request signal is independently transmitted using a dedicated resource. Accordingly, an occasion where a user should simultaneously transmit the scheduling request signal and ACK/NACK signal does not frequently occur. However, particularly, in the case where downlink data is frequently transmitted and no uplink data is transmitted, if uplink data transmission is urgently needed, probability of occurring such an occasion is further higher. This will invite a problem in transmission of an uplink control channel and give a negative affect to downlink and uplink throughputs. In such a situation, there are largely two methods for requesting scheduling.
First, there is a method of requesting uplink scheduling in a contention-free scheme using a scheduling request signal channel. Such a method is referred to as a multi-code transmission method or a multi-sequence transmission method. That is, it refers to a method of simultaneously transmitting a plurality of independent signals using one or more sequences by a user. However, although scheduling request signals are simultaneously transmitted using a separated scheduling request signal channel, if a plurality of ACK/NACK signals is transmitted to uplink, characteristics of an uplink single carrier are damaged, and thus PAPR is increased as a result.
Second, there is a method of requesting scheduling by performing Time Division Multiplexing (TDM) or joint coding on an ACK/NACK signal and a scheduling request signal in the unit of a symbol or slot. However, when two or more signals having different error requirements are simultaneously coded and transmitted, all signals ultimately should follow a power setting value of a signal that has the strictest requirement, and thus there may be a problem of wasting power. Although such a power wasting problem can be solved in a variety of methods, a plurality of signals should be transmitted in one subframe, and thus it is certain that power consumption increases compared with a case where one signal is transmitted in one subframe.
Accordingly, it is required to transmit a scheduling request signal efficiently using an ACK/NACK channel and CQI channel, which are existing control channels. In order to operate the scheduling request signal on an existing control channel, compatibility of the control channel with other control signals should be considered. In addition, capacity of a user equipment capable of transmitting a scheduling request signal should be also considered.
Accordingly, an efficient control channel structure is needed to transmit a scheduling request signal.